Nondestructive testing of coatings using ultrasonic leaky Rayleigh waves

In an earlier work the authors successfully applied the leaky Lamb wave (LLW) technique to the measurement of elastic wave speeds in both the substrate and coating. In the present study, the leaky Rayleigh wave (LRW) technique is used to measure elastic wave speeds of the coating. In the LRW measurement, the specimen is immersed in water and the coated side is insonified obliquely by ultrasonic waves emitted from a transducer. A second transducer receives the reflected signals in a pitch-catch position. Unlike the LLW measurement, the reflection from the bottom of the substrate is gated out and the interaction between the specular reflection and the guided wave in the coating layer is optimized. For typical plasma sprayed samples, with coating thickness 0.1 mm {minus}0.3 mm, the amplitude spectrum of the recorded signals was found to have two adjacent well developed minima in the frequency range of 3--7 MHz. A theoretical model of this feature has enabled the determination of the longitudinal and transverse wave speeds of the coating.